package com.example.ui;

import com.example.factory.BeanFactory;
import com.example.service.IAccountService;

/**
 * 模拟一个表现层，用于调用业务层
 */
public class Client {
    public static void main(String[] args) {
        IAccountService as= (IAccountService) BeanFactory.getBean("accountService");
        as.saveAccount();
    }
}

package com.example.factory;

import java.io.IOException;
import java.io.InputStream;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;

/**
 * 一个创建Bean对象的工厂
 *
 * Bean：在计算机英语中，有可重用组件的含义。
 * JavaBean：用java语言编写的可重用组件。
 *
 *              javabean > 实体类
 *
 * 他就是创建我们的service和dao对象的
 *
 * 第一个：需要一个配置文件来配置我们的service和dao
 *      配置的内容：唯一标识=全限定类名（key=value）
 * 第二个：通过读取配置文件中的内容，反射创建对象
 *
 * 我的配置文件可以是xml，也可以是properties
 */
public class BeanFactory {
    //定义一个properties对象
    private static Properties props;
    //定义一个Map,用于存放我们要创建的对象，我们把它称之为容器
    private static Map<String,Object> beans;
    //使用静态代码块为Properties对象赋值
    private static boolean flag;
    static{
        //设置初始化状态为正在初始化
        flag=true;
        //实例化对象
        props=new Properties();
        //实例化容器
        beans=new HashMap<String,Object>();
        //获取properties对象的流对象
        InputStream in= BeanFactory.class.getClassLoader().getResourceAsStream("Bean.properties");
        try {
            props.load(in);
        } catch (IOException e) {
            throw new ExceptionInInitializerError("初始化properties失败");
        }
        //获取枚举对象
        Enumeration keys = props.keys();
        while(keys.hasMoreElements()){
            //获取bean对象的key
            String key = keys.nextElement().toString();
            //把bean对象放入beans容器中
            beans.put(key,getBean(key));
        }
        //设置初始化状态为已初始化
        flag=false;
    }

    /**
     * 根据bean的名称获取bean对象
     * @param beanName
     * @return
     */
    public static Object getBean(String beanName){
        Object bean= beans.get(beanName);
        //以下if语句解决初始化顺序带来的隐患，flag标识初始化中
        if(bean==null&&flag){
            String beanPath = props.getProperty(beanName);
            try {
                bean = Class.forName(beanPath).newInstance();
            } catch (InstantiationException|IllegalAccessException|ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
        return bean;
    }
}

package com.example.service;

/**
 * 账户业务层接口
 */
public interface IAccountService {

    /**
     * 模拟保存账户
     */
    void saveAccount();
}

package com.example.service.impl;

import com.example.factory.BeanFactory;
import com.example.service.IAccountService;
import com.example.dao.IAccountDao;

/**
 * 账户的业务层实现类
 */
public class AccountServiceImpl implements IAccountService {

    private IAccountDao accountDao= (IAccountDao) BeanFactory.getBean("accountDao");

    @Override
    public void saveAccount() {
        accountDao.saveAccount();
    }
}

package com.example.dao;

/**
 * 账户的持久层接口
 */
public interface IAccountDao {

    /**
     * 模拟保存账户
     */
    void saveAccount();
}

package com.example.dao.impl;

import com.example.dao.IAccountDao;

/**
 * 账户的持久层实现类
 */
public class AccountDaoImpl implements IAccountDao {
    @Override
    public void saveAccount(){
        System.out.println("保存了账户");
    }
}

accountDao=com.example.dao.impl.AccountDaoImpl
accountService=com.example.service.impl.AccountServiceImpl

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>learn</artifactId>
    <version>1.0-SNAPSHOT</version>

    <packaging>jar</packaging>


    <dependencies>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <version>1.4.200</version>
        </dependency>
    </dependencies>

    <repositories>
        <repository>
            <id>alimaven</id>
            <name>aliyun maven</name>
            <url>http://maven.aliyun.com/nexus/content/groups/public/</url>
            <releases>
                <enabled>true</enabled>
            </releases>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </repository>
    </repositories>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>14</java.version>
        <maven.compiler.source>14</maven.compiler.source>
        <maven.compiler.target>14</maven.compiler.target>
        <encoding>UTF-8</encoding>
    </properties>
</project>